AC contactors, also known as electromagnetic switches, have been widely employed in the application of power control. The operating principle of AC contactor is attained by providing current to the windings of the contactor to induce a magnetic field in order to manipulate the contact switch of the AC contactor to open or close, thereby controlling the power appliance. Because the AC contactor is durable in large-current applications where the maximum allowable current is as high as 800 Ampere, and is able to readily and frequently turn on or off the power supply for high-current power appliance, e.g. 380 VAC, the AC contactor is commonly used to control the electromagnetic motor with high startup current. The AC contactor can also be employed to control factory equipment, electric heater, metal working machine, or auto-control electric apparatus, thereby fulfilling the purpose of remotely controlling the power appliance or automatically controlling the power appliance.
Referring to FIG. 1, the driving circuit for the conventional AC contactor is illustrated. In FIG. 1, the AC contactor includes a winding M and a contact switch a, such as a normally-open switch, in which the contact switch a is connected in series with the power appliance 12 and the winding M is connected in series with a switch K. When the control circuit 11 turns on the switch K, the AC voltage Vac is applied to the winding M of the AC contactor through the switch K, thereby inducing a magnetic field. Under this condition, the contact switch a is close, and thus the AC voltage Vac is applied to the power appliance 12 through the contact switch a to power the power appliance 12. On the contrary, when the control circuit 11 manipulates the switch K to turn off, the AC voltage Vac can not be applied to the winding M through the switch K. Under this condition, the contact switch a will open to prevent the AC voltage Vac from being applied to the power appliance 12 through the contact switch a. Therefore, the power appliance 12 will cease operating.
However, there is an increasing trend for the auto-control system and power appliance to use DC voltage as the operating voltage. If the driving circuit for conventional AC contactor replaces the AC voltage Vac with a DC voltage with the same magnitude, e.g. 380 VDC, the winding M of the AC contactor will burn down due to overcurrent condition when the winding M of the AC contactor is driven by the DC voltage. This is because the pick-up voltage of the AC contactor is high. If the winding M is driven by a DC voltage having the same magnitude of the AC voltage Vac, the winding M will reach saturation with small DC resistance as the winding M is steadily magnetizing. Therefore, the winding M will undergo over-current conditions and burn down accordingly. Thus, the auto-control system and power appliance using the DC voltage as the driving voltage generally employ DC contactors with higher price.
For example, the emergency power supply using DC voltage in a power system requires an additional inverter for generating an AC voltage if an AC contactor is used to control the circuitry of the emergency power supply, thereby driving the winding to operate normally. The additional inverter will increase the size of the emergency power supply and lower the power efficiency of the emergency power supply. What is worse, the additional inverter will cause a loud noise and increase the cost of the emergency power supply.
Therefore, it is necessary to address the foregoing deficiencies encountered by the conventional driving circuit for AC contactor.